When adding Avatar email-hash pairs we simply want the DB table to
represent a Set. We don't care if the hash-pair is already present,
so we just simply Insert and ignore the error.

Unfortunately this seems to cause some DBs to log the duplicate
insert to their logs - looking like a bug a in Gitea.

Now, there is no standard way in SQL to say Insert but if there's
an error ignore it. MySQL has INSERT IGNORE, PostgreSQL >= 9.5 has
INSERT ... ON CONFLICT DO NOTHING, but I do not believe that SQLite
or MSSQL have variants.

This PR places the insert in a transaction which we are happy to fail
if there is an error - hopefully this will stop the unnecessary
logging.

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

speed up page generation by making avatar lookup occur at the browser
not at page generation

* Protect against evil email address ".."

* hash the complete email address

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-Authored-By: Lauris BH <lauris@nix.lv>